The present invention concerns storage methods and associated devices for cryopreservation of cells, such as mammalian cells, and tissue samples/specimen.
Cells and tissues are frequently cryopreserved to temporally extend their viability and usefulness in biomedical applications. The process of cryopreservation involves, in part, placing cells into aqueous solutions containing electrolytes and chemical compounds that protect the cells during the freezing process (cryoprotectants). Such cryoprotectants are often small molecular weight molecules such as glycerol, propylene glycol, ethylene glycol or dimethyl sulfoxide (DMSO).
As these solutions are cooled to temperatures slightly below their freezing point, the solution remains in the liquid state. This condition in which the solution remains liquid below its phase transition temperature is termed supercooling. As the aqueous solutions are cooled further below their freezing point, the extent of supercooling increases. In the absence of intervention, the water molecules in the solution will, at a point usually no more than 15.degree. C. below the freezing point, spontaneously crystallize, and pure water will precipitate as ice.
During this transition from the liquid to the solid state, the solution moves from a higher to a lower free energy state, resulting in an exothermic reaction. The heat produced during this phase transition causes a transient warming of the sample during which the sample temperature increases. Meanwhile the surrounding environment (e.g. the device in which the sample is being cryopreserved) either remains at a constant temperature or continues to cool (depending upon the cooling approach used). Subsequently, as the heat in the sample dissipates, the thermal dis-equilibrium between the sample and cooling device created during this event causes the sample to undergo a rapid cooling rate to re-establish thermal equilibrium. In many cases this rapid cooling rate causes the formation of intracellular ice, which usually results in cell death. This formation of intracellular ice is typically dependent upon the mass of the sample, the heat transfer properties of the sample container, the cooling protocol used and the fundamental cryobiological properties of the cells.
The relationship between the frozen state and living systems has been fascinating mankind for years. As early as 1683, Robert Boyle observed that some fish and frogs could survive sub-freezing temperatures for short periods of time if a fraction of their body water remained unfrozen. Artificially induced cryopreservation was first observed in 1948 by Polge, Smith, and Parkes by the serendipitous discovery of the cryo-protective properties of glycerol for fowl and bull semen and, subsequently, for red blood cells. In more recent times, scientists interested in the natural phenomena and biomedical applications associated with freezing biological systems have begun to investigate the fundamental processes governing the relationship. To begin with, it is well known that decreased temperature results in the suppression of metabolic activity and, thus, in a reduction of the rate at which deterioration of an unnourished biological system would occur. The freezing process, however, is not as benign as one might assume; it generally induces extreme variations in chemical, thermal, and electrical properties that could be expected to alter intracellular organelles, cellular membranes and the delicate cell-cell interaction systems associated with tissues and organs. Indeed, given the extreme complexity of even the simplest biological cells, it is therefore remarkable that a reversible state of suspended animation by freezing is possible at all.
Since that first discovery of the cryoprotective effects of glycerol and the subsequent discovery of the widely applicable permeating cryoprotectant dimethyl sulfoxide (DMSO)), many investigators have attempted the preservation of cells or tissues, mostly through empirical methods. Most cell suspension cryopreservation protocols have been established using molar concentrations of permeating cryoprotective additives to enable freezing survival. By using these artificial cryoprotectants, much flexibility has been added to the cryopreservation process. For example, human red blood cells need to be cooled at a rate of around 1000.degree. C./min. for optimal survival without the addition of a cryoprotective agent (CPA). In the presence of 3.3M (30%) glycerol, however, survival of this cell type remains around 90% over a 2-3 log range in cooling rates. As can be expected, the higher the CPA concentration, the greater the likelihood of osmotic damage during the addition/removal of the substance, and consequently the greater care that is necessary in these processes.
During any cryopreservation process, the solutions involved will supercool below their freezing point until they find a random nucleation site for crystal formation. When cryopreserving by a freeze-thaw method, ice formation in the extracellular medium should be deliberately initiated by seeding at low degrees of supercooling. If ice formation is not induced by seeding, ice will form spontaneously when the solution is cooled sufficiently far below its equilibrium freezing point. Because this process is random in nature, ice formation will occur at random, unpredictable temperatures; consequently, sample survival rates will be highly variable between repeated trials with the same freezing protocol. Furthermore, the extremely rapid crystallization which results when ice forms in a highly supercooled solution causes damage to cells and tissues. Moreover, it has been shown that if extracellular ice formation is initiated at high degrees of supercooling, the probability of damaging intracellular ice formation is drastically increased. This phenomenon results from the delayed onset of freeze-induced cell dehydration, which results in increased retention of intracellular water, and thus higher likelihood of ice formation in the cell.
As noted above, during the transition from the liquid to the solid state, the solution moves from a higher to a lower free energy state which results in thermal disequilibrium between the sample that continues to warm and the cooling device that continues to cool. This disequilibrium ultimately results in a severe deviation from the cooling rate prescribed for the particular cell type, and the potential for cell damage during the process.
To prevent these potentially damaging situations from occurring, steps in the cryopreservation process often include interventions to introduce ice crystals in the extracellular solution near the solution freezing point. This process called “seeding” is typically performed by cooling the samples to near the solution freezing point, then touching the outside of the sample container with a metal device (e.g. forceps or a metal rod) precooled in a cryogenic fluid (e.g. liquid nitrogen). This seeding step produces ice crystals in the extracellular solution and provides a “template” upon which supercooled water molecules in the solution organize and produce further ice. However, seeding samples in this manner is time consuming and places the samples at risk in cases where they are temporarily removed from the cooling device for this procedure and because this method of seeding may inadvertently cause intracellular ice formation.
There is a need for a cryopreservation system that avoids the problems associated with the disequilibrium conditions described above. There is a further need for such a system that does not require the ancillary seeding step currently conducted to induce controlled ice crystal production. There is an additional need for a cryopreservation device that facilitates the solution to the above-noted problems. The needed cryopreservation device should also provide means to simplify its use in acquiring and storing cells and tissue to be cryopreserved.